1. Field of the Invention
The present invention relates to a thermal recording apparatus for recording an image or other information on a thermosensitive recording medium with a plurality of laser beams emitted from a plurality of laser beam generating devices.
2. Description of the Related Art
Thermosensitive recording apparatus for applying thermal energy to a thermosensitive recording medium to record an image or other information thereon are in wide use. Particularly, thermosensitive recording apparatus which employ a laser output source as a thermal energy source for high-speed recording are known from Japanese laid-open patent publications Nos. 50-23617, 58-94494, 62-77983, and 62-78964, for example.
The applicant has developed a thermosensitive recording medium capable of recording a high-quality image for use in such thermosensitive recording apparatus. The thermosensitive recording medium comprises a support coated with leuco dye, a color developer, and light-absorbing dyes, and produces color whose density depends on the thermal energy that is applied to the thermosensitive recording medium. The applicant has also developed an apparatus for recording an image or other information on the thermosensitive recording medium with a laser beam. For details, reference should be made to Japanese patent applications Nos. 3-62684 and 3-187494.
The thermosensitive recording medium has a thermosensitive layer on the support. The thermosensitive layer is produced by coating a coating solution on the support. The coating solution contains an emulsion which is prepared by dissolving thermally fusible microcapsules containing at least leuco dye, a color developer, and light-absorbing dyes into an organic solvent that is either slightly water-soluble or water-insoluble, and then emulsifying and dispersing the dissolved materials.
The leuco dye produces color by donating electrons or accepting protons from an acid or the like. The basic dye precursor comprises a compound which is normally substantially colorless and has a partial skeleton of lactone, lactam, sultone, spiropyran, ester, amide, or the like, which can be split or cleaved upon contact with the color developer. Specifically, the compound may be crystal violet lactone, benzoil leucomethylene blue, malachite green lactone, rhodamine B lactam, 1,3,3-trimethyl-6'-ethyl-8'-butoxyindolino-benzospiropyran, or the like.
The color developer for the leuco dye may be of an acid substance such as a phenolic compound, an organic acid or its metal salt, oxybenzoate, or the like. The color developer should preferably have a melting point ranging from 50.degree. C. to 250.degree. C. Particularly, it should be of a slightly water-soluble phenol or organic acid having a melting point ranging from 60.degree. C. to 200.degree. C. Specific examples of the color developer are disclosed in Japanese laid-open patent publication No. 61-291183.
The light-absorbing dyes should preferably comprise dyes which absorb less light in a visible spectral range and have a particularly high rate of absorption of radiation in an infrared spectral range. Examples of such dyes are cyanine dyes, phthalocyanine dyes, pyrylium and thiopyrylium dyes, azulenium dyes, squarylium dyes, metal complex dyes containing Ni, Cr, etc., naphtoquinone and anthraquinone dyes, indophenol dyes, indoaniline dyes, triphenylmethane dyes, triallylmethane dyes, aminium and diimmonium dyes, nitroso compounds, etc. Of these dye materials, those which have a high radiation absorption rate in a near-infrared spectral range whose wavelength ranges from 700 nm to 900 nm are particularly preferable in view of the fact that practical semiconductor lasers have been developed for generating near-infrared laser radiation.
In order to keep the thermosensitive recording medium in stable storage, the thermosensitive recording medium is designed such that it does not produce color with thermal energy whose level is lower than a certain threshold value. Therefore, the laser output source is required to produce a considerable thermal energy for enabling the thermosensitive recording medium to produce a desired color.
When the thermosensitive recording medium is thermally fused, the materials thereof are dispersed to react with each other for producing a color. Consequently, it is preferable to maintain the thermosensitive recording medium at a color-producing temperature for a certain period of time.
However, since the thermosensitive recording medium has heretofore been scanned with a single laser beam having a Gaussian distribution, each pixel on the thermosensitive recording medium undergoes an abrupt temperature change owing to the Gaussian beam. Therefore, it has been difficult to produce color with a stable density on the thermosensitive recording medium.